1. Field of the Invention
The present invention relates to a burner combustion control system which may be effectively applied, for example, to a radiant tube burner having a double-casing structure.
2. Description of Prior Art
It is desirable for a burner that burns a liquid or gaseous fuel to maintain an optimal combustion state during the burning operation. Prior arts for such combustion control include one in which the intensity of light emitted from the flame of a burner is detected in the form of an electric signal by using a semiconductor device, e.g., a phototransistor, a photodiode or a solar cell, and combustion control is effected by use of the integral value of a power spectrum that is obtained by frequency analysis of an oscillating waveform of light from the flame on the basis of the output current from such a semiconductor device. According to another prior art, the intensity of light emitted from the burner flame is detected in the form of an ionic current by using an electrode that is inserted into the flame in place of the sensor. The former prior art, that is, the method wherein light emitted from the flame is detected as an electric signal and this signal is processed in an electric circuit, uses a semiconductor photosensor, e.g., a phototransistor, a photodiode, etc., to detect the light from the flame. In this case, if the photosensor is not provided at a proper position, favorable control may not be realized.
Under these circumstances, the present inventor developed and proposed a technique for solving the above-described problem (see Japanese Patent Laid-Open (KOKAI) No. 3-170717). With this technique, oscillating light from the combustion flame is detected with a photosensor, which is provided on the side opposite to the burner, that is, in the rear part of the furnace because it is necessary to detect the whole turbulent burning portion of the flame even when the flame length changes owing to turndown control. However, there are cases where the photosensor cannot be attached to the side opposite to the burner because of the furnace structure; therefore, the desired effect cannot always be expected.
Japanese Patent Laid-Open (KOKAI) No. 3-170718 discloses a combustion control system which is free from the above-described problems. In this invention, an electrode is inserted into the flame from the burner nozzle side to detect a flame ionic oscillating current, thereby making the control system free from the restriction by the furnace structure. However, even such a control system, in which an ionic current is detected with an electrode, is inadequate for some burners, for example, radiant tube burners, wherein flame is formed inside the inner casing of a double-casing structure and radiation from the outer casing is utilized. That is, since such burners are arranged to suck combustion gas from an exhaust port by using an ejector or the like in order to enable continuous combustion, the flame point changes owing to turndown control, so that detection cannot be effected for a specific portion of the flame even with an electrode.
FIG. 7 schematically shows related art (Japanese Patent Application Laid Open No. Hei 4-68213) which is an improvement over the combustion control system for the burner combustor of the type described above and on which the invention of this application is based.
FIG. 7 will be explained below. The burner has a double-casing structure comprising an outer casing 1 and an inner casing 2, and a gas nozzle 3 is provided inside the inner casing 2. A fuel supply pipe 5 is connected to the gas nozzle 3 through a fuel shut-off valve 4 to supply fuel 6 to the nozzle 3. The proximal end portion of the inner casing 2 is provided with an inspection port 7, and a lens 8 is set in the port 7, thereby enabling flame 9 in the inner casing 2 to be monitored.
An optical fiber bundle 11 is disposed such that one end thereof is coincident with a focal point 10 of the lens 8, and a photosensor 12 is attached to the other end of the optical fiber bundle 11. A signal output from the photosensor 12 is amplified in an amplifier 13, rectified in a rectifier 14, integrated in an integrator 15 and then input to a low-limit comparator 16 and a high-limit comparator 17. The low- and high-limit comparators 16 and 17 compare the signal from the integrator 15 with a low-limit voltage 18 and a high-limit voltage 19, respectively. When the signal from the integrator 15 is lower than the low-limit voltage 18, the low-limit comparator 16 turns on, whereas, when the signal from the integrator 15 exceeds the high-limit voltage 19, the high-limit comparator 17 turns on. When either of the comparators 16 and 17 turns on, it introduces the output signal from the integrator 15 to a logical operator 20 in the subsequent stage, the output terminal of the operator 20 being connected to the fuel shut-off valve 4. In the figure, reference numeral 21 denotes an exhaust gas discharge section, 22 combustion gas, 23 an air supply pipe, and 24 combustion air. In addition, a swirler 25 is attached to the distal end portion of the gas nozzle 3.
In the combustion controller for the combustor, the inside of the inner casing 2 is monitored with the photosensor 12 through the lens 8 and the optical fiber bundle 11. A signal output from the photosensor 12 is input to the logical operator 20 through the light oscillating power detecting circuit to output a signal for the open-close control of the fuel shut-off valve 4 in cooperation with the output signal from the integrator 15.
The above-described prior art suffers from some problems stated below: In the above combustion controller, a value output from the integrator 15, that is, a voltage obtained by rectifying and integrating the light oscillating signal, is compared with the low- and high-limit voltages 18 and 19 in the low- and high-limit comparators 16 and 17, respectively, and since these reference voltages are set in a hardware manner, each can be set only at a single reference point, which gives rise to problems: For example, in the case of a burner with a turndown ratio, the above-described integrated voltage varies to a great extent according to the burning rate and the reference voltage employed for detection of incomplete combustion also varies with the burning rate; therefore, the above-described combustion controller cannot be used for a burner with a turndown ratio.
In addition, it is possible to adjust the time constant of the integrator 15 and the low- and high-limit voltages 18 and 19 used as references in conformity to a burner to which the above-described control system is to be applied: for example, in the case of the time constant of the integrator 15, the capacitor is replaced with another to adjust variations in the detection time and the integrated voltage. However, since the capacitance of each individual capacitor is standardized, it is not always possible to obtain an optimal capacitance. In addition, since the controller is an analog circuit, it is extremely difficult to effect fine adjustment by changing capacitors.
In view of the above-described circumstances, it is an object of the present invention to provide a burner combustion control system which is applicable to even a burner with a turndown ratio and which has a detector capable of detecting a combustion state of the burner accurately and also enables a target combustion state to be maintained on the basis of the result of the detection.